Cancelled number still being billed

I have cancelled a number on my dads contract in April already. They keep on billing me and then also cut all the phones because of none payment after I phone them every month to query the cancelled number. According to my knowledge they owe me a credit and therefor I will not pay the account. This is the worst service provider I have ever seen in my life. You send email after email and all that happens is that you get a reference number and then about two weeks after that the call gets resolved but we are still being billed for the cancelled number. SHOCKING SERVICE!!!!!!